Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Roaring Pipe

Objective

To understand vibration of air column in a pipe open at both ends.

Introduction

Vibrations of air column in a pipe open at both ends consists of fundamental frequency together with all harmonics.

Apparatus

internally corrugated flexible plastic pipe of about one metre length.

Procedure

Hold one end of the pipe and whirl it in a circle. It creates a musical sound. Change the speed and length of the portion which is rotating, sound also changes.

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Discussion

Air pressure is decreased at the rotating end (Bernoulli principle). So air enters from the end in your hand and exits from the rotating end. The faster you rotate, the faster the air shoots out.

Corrugations cause the passing air to vibrate. These vibrations strike your eardrums and you hear the sound. Whirl the pipe faster, the sound is of higher frequency. Slow whirling lowers the frequency.

To verify that air really passes through the rotating pipe, hold tiny paper bits near the end in your hand. These papers bits are sucked up and comes out through the rotating end.